It is well known that a multipath disturbance is generated in an FM radio receiver for an automobile, and that this multipath disturbance is caused by a multipath distortion of a demodulated signal of a received wave, which is generated by interference between a direct wave of a radiowave and a reflected wave thereof reflected by a mountain, a building, or the like. This multipath distortion degrades the sound quality of a demodulated aural signal.
As shown in FIG. 3, multipath distortion is a high frequency instantaneous noise intermittently superimposed on a demodulated signal. To remove this multipath distortion, if a demodulated signal is passed through a low-pass filter during generation of the multipath disturbance, the multipath distortion mainly consisting of a high frequency component is reduced and cannot be aurally detected.
Conventionally, an apparatus using a variable low-pass filter is known as a noise reduction apparatus. In this apparatus, a demodulated signal is passed through a variable low-pass filter and a noise detecting circuit is provided for outputting a detection signal throughout a period in which a multipath distortion is intermittently generated when the intermittent multipath distortion is superimposed on the demodulated signal. When the multipath distortion is detected, a cut-off frequency of the variable low-pass filter is set at a fixed low frequency value in response to a detection signal from the noise detecting circuit, to remove the multipath distortion consisting of the high frequency component. When a multipath distortion is not detected, the cut-off frequency is set at a high frequency to pass the demodulated signal without digradation.
This noise reduction apparatus removes the high frequency component of an input signal throughout the entire period in which a noise is intermittently generated, and the high frequency component of the input signal is inevitably degraded when the noise reduction ratio is increased.
The following relationship is generally present between noise such as a multipath distortion and a signal on which the noise is superimposed.
When the signal does not include a high frequency component, a high frequency noise stands out, but when the signal includes a high frequency component, the noise does not stand out. Therefore, in the former case, the noise must be removed, but in the latter case, the removal of the noise is not necessary and only significant noise need be removed, i.e., noise need be removed only slightly.
Since noise is normally at a constant level, the S/N ratio is degraded when a signal level is low and is not degraded when the signal level is high. Therefore, in the former case, noise must be removed, and in the latter case, noise need be removed only slightly. Assuming that a signal level is constant, the S/N ratio is degraded when a noise level is high and is not degraded when the noise level is low. Therefore, in the former case, noise must be removed, and in the latter case, noise need be removed only slightly.
However, a conventional noise reduction apparatus as described above does not consider such a relationship between a signal and noise, and always carries out a noise reduction processing with a consistent characteristic, i.e., processing wherein a cut-off frequency of a variable low-pass filter is set at a fixed low frequency and a signal is passed through the variable filter, and consequently, does not perform a proper noise reduction.
Accordingly, it is an object of the present invention to provide a noise reduction apparatus capable of varying a noise reduction characteristic in accordance with a frequency or a level of a signal to be subjected to noise reduction, thereby effectively removing noise.